WASHINGTON (Reuters) - The Interior Department's inspector general said it would investigate a reported threat by the agency's head, Ryan Zinke, against Republican Senator Lisa Murkowski's state of Alaska last week over her opposition to her party's efforts to repeal Obamacare.

Zinke called Murkowski and Alaska's other U.S. senator, Republican Dan Sullivan, on July 26, the day after Murkowski's crucial vote against a motion to begin debate on the healthcare overhaul, the Alaska Dispatch News reported.

The interior secretary warned the two senators that Murkowski's vote could have negative consequences for energy and land use in Alaska, the newspaper said. Murkowski spokeswoman Karina Peterson confirmed that Zinke had called the senator.

The Interior Department deals with policies crucial to the state's economy such as drilling and mining on federal and tribal land and control of wildlife areas.

The inspector general's office sent a letter to Democratic U.S. Representatives Frank Pallone and Raul Grijalva, the top Democrats on the House committees on energy and natural resources, respectively, on Thursday evening, saying it would start a preliminary investigation into the matter.

The two lawmakers had asked the inspector general's office and the Government Accountability Office (GAO) for an investigation following the report of the threat.

Zinke's office and the GAO did not immediately respond to requests for comment.

Murkowski's July 25 no vote, along with that of Republican Senator Susan Collins of Maine, forced Vice President Mike Pence to cast a tiebreaking vote to pass the motion by a 51-50 ballot.

Subsequent efforts in the Senate to repeal the Affordable Care Act, commonly known as Obamacare, collapsed last Friday after Murkowski, Collins and Senator John McCain of Arizona joined their Democratic and independent Senate colleagues in voting against a "skinny repeal" bill that would have eliminated some parts of Obamacare.

Murkowski chairs the Senate Committee on Energy and Natural Resources, which oversees appropriations and appointee confirmations for Zinke's agency.

On Thursday, the interior secretary suggested he had reconciled with Murkowski, posting a photo of him drinking Alaska-brand beers with the senator on Twitter.